| Thermal power generation accounts for 68.5%of the national power generation.Coal power will still be the mainstay of our country's power supply system in the next few decades.The heat exchanger is an indispensable equipment for thermal power plants.The main material used for domestic heat exchangers is 316L austenitic stainless steel.However,316L stainless steel has shortcomings such as insufficient oxidation resistance,poor corrosion resistance,and insufficient durability.Ferritic stainless steel has excellent mechanical properties such as good ductility,low thermal expansion coefficient,high corrosion resistance and relatively low stress corrosion cracking sensitivity.Therefore,it is of great scientific significance and practical engineering value to find a new economical steel grade that can replace 316L stainless steel with excellent corrosion resistance.445J2 stainless steel is based on the traditional 445 stainless steel by adding trace amounts of Nb,Ti,Al,Mo And other alloying elements to prepare new economical stainless steel.In this paper,the pitting corrosion resistance of 445J2 stainless steel and 316L stainless steel is compared and studied.The pitting corrosion potential and corrosion current density of the two stainless steels at different temperatures and different Cl~-concentrations are compared through potentiodynamic polarization;Chemical experiments determined the critical pitting temperature of 445J2stainless steel and 316L stainless steel under 0.6 M NaCl,and analyzed the pitting corrosion starting point of 445J2 and 316L stainless steel by SEM and EDS,and studied the effect of different temperatures and different Cl~-concentrations on the points.The impact of eclipse.Then,through the IGC test,the resistance to intergranular corrosion of the two stainless steels at 550?-750?was compared;and the aging temperature range was further expanded.The DL-EPR experiment was used to study the different aging temperature at 450?-850?.The influence of 445J2 stainless steel's intergranular corrosion sensitivity;choose the aging temperature of 750?to compare the influence of different aging time on its intergranular corrosion sensitivity.SEM and EDS analysis of the composition of the precipitation at the grain boundaries of 445J2 stainless steel under different aging temperatures and aging times revealed the intergranular corrosion mechanism of 445J2 ultra-pure ferritic stainless steel.The main findings are as follows:(1)According to the potentiodynamic polarization and IGC test results,under the same experimental conditions the pitting resistance and intergranular corrosion resistance of 445J2stainless steel are significantly better than that of 316L stainless steel.The critical pitting corrosion of 445J2 and 316L stainless steel under 0.6 M NaCl The temperatures are 48?and20?,respectively.DL-EPR experimental results show that the intergranular corrosion sensitivity of 445J2 stainless steel first increases and then decreases with the increase of the aging temperature(450?-850?),and the DOS reaches the maximum value of 47.18%at650?.The intergranular corrosion sensitivity of 445J2 stainless steel at 750?increases first and then decreases with the increase of the aging time(15 min-200 min),and the DOS reaches the maximum value of 42.86%at 60 min.(2)According to the potentiodynamic polarization test and the corrosion morphology characterization results,As the temperature rises,the pitting corrosion potential of 445J2 and316L stainless steels decreases,and the corrosion current density Increase.With the increase of Cl~-concentration,the pitting corrosion potential of 316L stainless steel gradually decreases,and the pitting resistance ability decreases.With the increase of Cl~-concentration,445J2stainless steel will have a passivation film breakdown at a lower potential,but it will re-passivation immediately after breakdown at a high concentration of Cl~-.So,at the 0.6 M Cl~-concentration shows the largest pit size.(3)The two types of stainless steel have great differences in pitting morphology.Under the same conditions,the pitting corrosion of 316L stainless steel is more serious than that of445J2 stainless steel.There are more pitting corrosion starting points,and the pitting corrosion pits are connected to each other,and the largest pitting corrosion The pit reaches1043?m.445J2 stainless steel has better resistance to pitting corrosion than 316L stainless steel.This is because the passive film of 316L stainless steel has a poorer protection ability,and the corrosion products that produce small pitting corrosion are easy to peel off,making the pitting corrosion deeper and wider.Expand and connect with each other.The 445J2stainless steel passivation film shows good re-passivation ability,effectively inhibiting the expansion of pitting corrosion.(4)Intergranular corrosion mechanism of 445J2 ultra-pure ferritic stainless steel with stable elements such as Nb,Ti and Mo:in the early stage of aging,C atoms diffuse to the grain boundaries first,and then,because most of the Nb/Mo has not diffused to the grain boundaries,the precipitates at the grain boundaries are mainly Cr-rich carbides.As the aging time increases,Nb/Mo diffuses to the grain boundary to replace the Cr in the carbide on the grain boundary,so that the Cr content at the grain boundary is supplemented,and the intergranular corrosion sensitivity is reduced. |
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